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Organometallics 1991,10, 3150-3155
drogens of the ethylene close to the two nearest-neighbor Pt atom. "he diosmacyclobutane 1 is thus not only a good vibrational3 model for ethylene chemisorbed on Pt(ll1) but a good structural one as well. Acknowledgment. This research was supported by the Department of Energy Grant DE-FG02-84ER13299-AO6. Los Alamos National Laboratory is operated by the University of California for the Department of Energy
under Contract W-7405-ENG-36. We are grateful to Colonial Metals, Inc., and to Degussa Corp. for the loan of Os04, and to Professor Lawrence Bartell for helpful discussions. Supplementary Material Available: A stereoview of the unit cell for 1 (1 page); a listing of observed neutron structure factor amplitudes (31 pages). Ordering information is given on
any current masthead page.
Theoretical Study of the Sandwich Compounds commo-3,3'-Si(3,1 ,2-SiC2BgHll), and commo-1,l'-Si( l,2,3-S/C2B4H& John A. Maguire Chemistry Department, Southem Methodist Univers& Dallas, Texas 75275
Recelved January 23, 199 1 A theoretical study of the sandwich compounds comm0-3,3'-Si(3,1,2-SiC~BgH11)~ (I) and commo-1,l'Si(1,2,3-SiCzB4H6)2 (11) was conducted by using MNDO-SCF semiempirical molecular orbital theory. In agreement with experiment, the calculations show that the silicon atoms in both I and I1 are slipped toward the boron sides of the C2B3 carborane bonding faces. This slip distortion was found to be the result of Si(s)-cage carbon antibonding interactions and an enhancement in intracage bonding on slippage. Preference for a Si(1V) oxidation state in I and I1 and a Si(I1) state in (q5-C5H5)2Si (111) could be explained on the basis of strong antibonding interactions in the LUMO's of I and I1 that are not present in the HOMO of 111. A comparison of the MNDO results with Xa-scattered wave SCF molecular orbital calculations in which silicon harmonics up through 1 = 2 were used for I1 and I11 indicates that Si(d) orbital participation is not important in complex bonding.
Introduction The number of main-group metallacarboranes that have been synthesized and structurally characterized has increased greatly in the past 5 years. Most of these compounds result from the reaction of a metal ion, or metalcontaining group, with the dianions nido-1,2-C2BsHll2-, nid0-2,3-C~B~H~~-, or their derivatives.'v2 Depending on the metal and its oxidation state, the heteroatom can be incorporated into the polyhedral structure of the carborane to form either the respective closo or the commo complexes. In the icosahedral system, structures of the commo complexes of Al(III),3t4Ga(III),6 and Si(IV)6 have been reported, while, in the pentagonal-bipyramidal system, the structures of the commo complexes of Si(IV)7*8and Ge(1)Hosmane, N. S.;Maguire, J. A. In Molecular Structureand Energetics; Liebman, J. F.,Greenberg, A., Willinma, R. E.,We.;VCH New York, 1988; Vol. 5, pp 297-328. (2)Hoemane, N. 5.;Maguire, J. A. Adu. Organomet. Chem. 1990,30, 9 9. ._
(3) Reee, W. S.,Jr.; Schubert, D. M.; Knobler, C. B.; Hawthorne, M. F.J . Am. Chem. SOC.1986,108,5367. (4)Bandman, M. A.;Knobler, C. B.; Hawthorne, M. F. Inorg. Chem.
27., __ 2899. -19RB. - --, -. - -. (5)Bandman, M. A.; Knobler, C. B.; Hawthorne, M. F. Inorg. Chem. 1989,28,1204. (6)(a) Reee, W. S.,Jr.; Schubert, D. M.; Knobler, C. B.; Hawthorne, M. F.J. Am. Chem. SOC.1986,108,5360.(b) Schubert, D.M.; Rees, W. S.,Jr.; Knobler, C. B.; Hawthorne, M. F. Organometallics 1990,9,2938. (7)Hoemane, N.S.;de Meeater, P.; Siwardane, U.; Islam, M. S.;Chu, S.S . C. J. Chem. SOC.,Chem. Commun. lB86, 1421. (8) Siriwardane, U.;Islam, M. S.;West, T.A.; Hoemane, N. S.; Maguire, J. A.; Cowley, A. H. J. Am. Chem. SOC.1987,109,4800.
0276-7333/91/2310-3150$02.50/0
(IV)>lowith ~ ~ ~ ~ O - Z [ S ~ ( C H ~ ) ~ I -(R ~ -= [H,R ] - ~ , ~ - C Si(CH3)3,CH3), are known. There have been numerous structural reports on closo1-Sn-2-Si(CH3)3-3-R-2,3-C2B4H4 (R = Si(CH3)3, CH3, H)'J1J2 and their adducts with monodentate,13 bidentate,12J4J5bis(bidentate),13and tridentate16 Lewis bases. Similar, though less extensive, studies have been reported for the analogous Ge(II)"J8 and Pb(II)1e*20metallacarboranes. In the icosahedral system, structural studies (9)Hosmane, N. S.;de Meeeter, P.; Siriwardane,U.; Islam,M. S.; Chu, S.S . C . J. Am. Chem. SOC.1986,108,6050. (10)Islam, M. S.;Siriwardane, U.; Hoemane, N. S.; Maguire, J. A.; de Meester, P.;Chu, S. S. C. Organometallics 1988,7,2340. (11)Cowley, A. H.; Galow, P.; Hosmane, N. S.;Jutzi, P.; Morman, N. C . J . Chem. SOC.,Chem. Commun. 1984,1504. (12)Hosmane, N. 5.; de Meester, P.; Malder, N. N.; Potts, S. B.; Chu,
S.S.C.;Herber, R. H. Organometallics 1986,5,772. (13)Hosmane, N. S.;Fagner, J. S.;Zhu,H.; Siriwardane, U.;Maguire, J. A,; Zhang, G.; Pinkston, B. S.Organometallics 1989,8,1769. (14)Siriwardane, U.; Hoemane, N. S.;Chu, S.S.C. Acta. Crystallogr.,
Sect. C 1987,C43, 1076. (15)Maguire, J. A.; Fegner, J. S.; Siriwardane, U.; Banewicz, J. J.; Hoemane, N. S. Struct. Chem. 1990,1,583. (16)Siriwardane, U.;Meeuire, J. A.; Banewicz, J. J.; Hoemane, N. S. Organometallics 1989,8,2792. (17)Hoemane. U.:Islam, M. S.;Pinkston. B. S.;Siriwardane.. U.:. Banewicz, J. J.; MGuire, J. A. Organometallics 1988,7, 2340. (18)Siriwardane, N. S.;Islam,M. S.;Maguire, J. A.; Hosmane, N. S. Organometallics 1988, 7, 1893. (19)Hoemane, N. S.;Siriwardane, U.; Zhu, H.; Zhang, G.; Maguire, J. A. Organometallics 1989,8,566. (20)Hosmane, N. S.; Lu, K.; Zhu, H.; Siriwardane, U.; Shet, M. S.; Maguire, J. A. Organometallics 1990,9,808.
0 1991 American Chemical Society
Theoretical Study of Two Sandwich Carboranes have been reported for the complexes l-Sn(X)-2,3(X= C4H80and Cl,$&).11.21 (CH3)2-2,3-C2BgHg All structures show that the metals bond to the CzB3 open faces of the carboranes. However, the metals are not necessarily symmetrically bonded to the atoms of these faces but are dislocated, or slipped, toward the boron side of the faces. The extent of slippage is a function of the metal, its coordination to other ligands, and the substituents on the cage atoms of the carborane.22 While there have been a number of theoretical studies on the maingroup closo-metallacarboranes and their Lewis base add ~ ~ t sthere , ~have ~ been J ~no ~parallel ~ ~ studies ~ ~ of commo-metallacarboranes. With the proper disclaimers, qualitative arguments based on cyclopentadienide (Cp-) orbitals have been used to explain the bonding in both the icosahedrals and pentagonal-bipyramidaP main-group commo-metallacarboranes. Although the analogy between the carborane and cyclopentadienide anions provided a useful guide in the original synthesis of metallacarborane sandwich compounds by Hawthorne and co-workerP and theoretical calculations by MingoP have also shown that the primary metal-binding orbitals of the two ligands are similar, striking structural and stoichiometric differences are found between the two classes of main-group metal sandwich complexes. In the group 14 cyclopentadienyl sandwich complexes, the metals are present in +2 oxidation states, and many of the complexes are bent so that the Cp rings are not paralle1.26*27On the other hand, in the analogous commo-metallacarboranes, the metals are invariably in their +4 oxidation states and are sandwiched between essentially parallel faces of the two carborane ligands. The characteristic distortion found in the metallacarboranes is a slippage of the metal toward the boron side of the bonding CzB3 face.’P2 In view of these differences, explanations of the geometries and stabilities of the commo-metallacarboranesbased on cyclopentadienide-type orbitals would be highly qualitative and of questionable use. Therefore, theoretical calculations were carried out [Si(CzBgH1l)z],and on comm0-3,3’-Si(3,1,2-SiC~B~H~~)~, [Si(C2B4Hs)2],by using commo-1,l’-Si(l,2,3-SiC~B,Hg)z, MNDO-SCF and Xa-scattered wave (Xa-SW) methods and are reported herein.
Calculations MNDO-SCF semiempirical molecular orbital calculations were carried out by using version 2.1028 of the MOPAC packagem on an IBM 3081 computer. Unless otherwise noted, all geometric parameters were completely optimized with a modifiedm Davidson-Fletcher-Powell algorithm3’ incorporated as a standard part of the program. The pa(21) Jutzi, p.; Galow, p.; Abu-Orabi, s.; Arif, A. M.; Cowley, A. H.; Norman, N. C. Organometallics 1987,6, 1024. (22) Maguire, J. A,; Ford, G. P.; Hosmane, N. S. Inorg. Chem. 1988, 27, 3354. (23) Barreto, R. D.; Fehlner, T. P.; Hoemane, N. S. Inorg. Chem. 1988, 27, 453. (24) Hawthorne, M. F.; Young, D. C.; Andrews, A. D.; Howe, D. V.;
Pilling, R. L.; Pith, A. D.; Raintjes, M.; Warren, L. J., Jr.; Wegner, P. A. J. Am. Chem. SOC.1968,90, 879. (25) (a) Mingos, D. M. P. J. Chem. SOC.,Dalton Trone. 1977,602. (b) Mingos, D. M. P.; Forsyth, M. I. J . Chem. SOC.,Chem. Commun. 1977, 605. (c) Calhorda, M. J.; Mingoe, D. M. P.; Welch, A. J. J. Organomet. Chem. 1982,228,309. (26) Jutzi, P. Ado. Organomet. Chem. 1986, 26, 217. (27) Jutzi, P.; Kanne, D.; Kruger, C. Angew. Chem. 1986, 25, 164. (28) Olivella, S. QCPE Bull. 1984, 4 , 109. (29) Stewart, J. P. QCPE Bull. 1983, 3, 43. (30) Weiner, P. K. Ph.D. Dissertation, University of Texas at Austin, 1975. __ . _. (31) Dwidon, W. C. Comput. J . 1986,10,406. Fletcher, R.; Powell, M. J. D. Ibcd. 1963,6, 163.
Organometallics, Vol. 10, No. 9, 1991 3151 rameters for carbon, boron, and hydrogen were those stored in the version. The older32parameters for silicon were used instead of the newer ones.= The former were found to give slightly better geometries than the latter. The starting geometry for Si(C2B9H11)2 was its reported X-ray structure! while that of Si(C2B4Hs)zwas the X-ray structure of its C ( ~ a g e ) - S i ( c H derivative.’*8 ~)~ In analyzing some of the MNDO results, the total energy, E , was partitioned as the s u m of one-center, EA,and two-center, Em, terms such thatN-%
C E A + C C Em A
A
